1. Field of the Invention
The present invention relates to refractive surgical methods using lasers and software-driven scanning mechanisms for utility corneal reshaping by procedures of photorefractive keratectomy(PRK) and laser in situ keratomileusis(LASIK).
2. Prior Art
Various lasers have been used for ophthalmic applications including the treatments of glaucoma, cataract and refractive surgery. For non-refractive treatments suitable laser wavelengths are in the ranges of visible to near infrared. They include: Nd:YAG (1064 nm), doubled-YAG (532 nm), argon (488, 514 nm), krypton (568, 647 nm), semiconductor lasers (630-690 nm and 780-860 nm) and tunable dye lasers (577-630 nm). For refractive surgeries (or corneal reshaping), ultraviolet (UV) ArF lasers (at 193 nm) have been used for large area surface corneal ablation in a process called photorefractive keratectomy(PRK). Corneal reshaping may also be performed by laser thermal coagulation currently conducted by Ho:YAG lasers which however, has very limited long term clinical results.
The existing ArF excimer lasers have drawbacks of being large in size and weight, using toxic gas and high maintenance cost.
The existing non-excimer UV laser systems include (a) argon-pumped Ti:sapphire and (b) fifth-harmonic of flash lamp pumped Nd:YAG. System (a) is limited by high-cost and the life-time of the argon laser which is a bulky gas laser. System (b) is patented by the present inventor, J. T. Lin, (U.S. Pat. No. 5,144,630). However, this system has a rather low overall UV energy conversion efficiency and the available Nd:YAG laser at high repetition rate. Diode-pumped Nd:YAG or Nd:YLF (DPY) have not yet been converted into the UV (210-213) nm ranges with useful energy level for PRK procedures. Moreover the DPY technology is limited by the high-cost of the pumping diode array and the output quality and pulsewidth of the Nd:YAG (or Nd:YLF) fundamental beam. To achieve useful UV power for PRK procedures , (100-200) mW, one should require the fundamental beam to have a very good beam quality (at least 90% Gaussian and beam divergence of smaller than 3 mrad), short pulse duration (less than 15 nanosecond) and high repetition rate (higher than 150 Hz).
In light of the above, it is an object of the present invention to provide refractive laser systems which offer the advantages of: low-cost, reduced size and weight, high reliability, easy to operate and maintain. Another object of this invention is to provide a computer-controlled scanning device which only requires low UV energy and such that all solid-state lasers becomes possible for use in PRK and other refractive surgeries currently performed only by excimer(ARF) laser and an Argon-laser-pumped Ti:sapphire laser.
Another object of this invention is to provide novel laser crystals and frequency up-conversion schemes producing good beam quality with short pulse duration and high repetition rate to achieve the required UV averaged power.
It is yet another object of the present invention to provide a refractive laser system which is compact, portable and insensitive to environmental conditions (such as vibration and temperature). This portable system may also be used in a mobile clinical center.
The prior U.S. Pat. No. 4,784,135 of Blum, et al. and assigned to IBM teaches the first use of ultraviolet irradiation (shorter than 200 nm) of a biological layer to cause ablative photo decomposition. This patent teaches that using a laser beam having a wavelength of 193 nm and an energy level of much greater than 10 mJ/pulse can be used to photoablate corneal tissue without the build up of excess heat. The present invention on the other hand proposes a much lower UV energy per pulse of (0.05-2) mJ on corneal surface for photoablation.
There are several prior art U.S. Patents relating to refractive surgery, or photorefractive keratectomy. U.S. Pat. No. 4,784,135 suggests the use of a UV laser with wavelengths less than 200 nm, in particular ArF laser at 193 nm, for non-thermal photoablation process in organic tissue. Devices for beam delivery and methods of corneal reshaping are disclosed in U.S. Pat. No. 5,019,074 using an erodible mask. Techniques for corneal reshaping by varying the size of the exposed region by iris or rotating disk are discussed in Marshall et al, xe2x80x9cPhotoablative Reprofiling of the Cornea Using an Excimer Laser: Photorefractive Keratectomyxe2x80x9d Vol. a, Lasers in Ophthalmology, pp. 21-48 (1986). Tangential corneal surface ablation using ArF excimer laser or harmonics of Nd:YAG laser (at 532 and 266 nm) is disclosed in U.S. Pat. No. 5,102,409.
These prior arts of ArF excimer lasers, however, require high UV energy of (100-300 mJ) per pulse from the laser cavity or (30-40) mJ per pulse delivered onto the corneal surface, where large area corneal ablation using a beam spot size of about (4-6) mm which gives an energy density of (120-200) mJ/cm2. Moreover, the existing high power ArF excimer lasers operating at a repetition rate of (5-50) Hz will limit the practical use of the tangential ablation concept which takes 5-10 minutes for a 5 diopter corneal correction in a 5-mm optical zone. The high energy requirement of the currently used ArF excimer laser has the problems of: high-cost (in system, erodible mask and gas cost), high-maintenance cost, large size/weight and the systems are sensitive to environmental conditions, such as temperature and moisture.
More recently, the present inventor, J. T. Lin, has proposed a compact miniature-excimer laser with energy/pulse of (2-4) mJ from cavity and (0.8-1.2) mJ on corneal surface by using a scanning device. This system, however, is still a gas laser and repetition rate is limited to 100 Hz. Maintenance of this ArF laser is quite involved and energy stability is poor.
The L""Esperance U.S. Pat. No. 4,665,913 proposed a scanning ArF laser which requires a complex apparatus to select a section of the beam which is substantially uniform in density within a substantially square spot xe2x80x9cdotxe2x80x9d. The overall efficiency would be less than 10% from the output of the laser window to the corneal surface. Based upon this patent, the successive sweep of the scan areas would always leave ridges between the sweeps. It should be noticed that this L""Esperance""s patent uses overlaps between each of the scanned areas to obtain the desired ablation profiles of myopic (or other) corrections. However, the ridges between each of the successive ablated areas are very difficult to avoid if within each scanned area the ablated profiles are not substantially uniform. In fact, one should expect a very rough surface on these ablated areas in addition to the regular ridges between each overlapped zones. One of the problems found in these teachings is that it required individual ablated areas to be substantially uniform and in a round or square shape. This is very difficult to achieve. Even if perfectly uniform, a square portion of a fundamental beam is produced using a complex apparatus for beam reshaping.
The L""Esperance U.S. Pat. No. 4,665,913 does not appear to have found a suitable scanning method or an effective method of selecting a perfect beam (with uniform density and well-defined shape) which would overcome the above-described difficulties and make the proposed teaching become practical in cost and design for any clinical uses. In fact, L""Esperance""s scanning method has been challenged by the Muller, U.S. Pat. No. 4,856,513, where the difficulties and problems of L""Esperance""s teachings are discussed.
It is therefore a further object of the present invention to provide a method and apparatus for corneal reshaping by using a software-driven new scanning patterns which does not require substantially uniform density or a specific spot shape. Contrary to L""Esperance""s teachings, which suggest that there should be a perfect boundary match among each square beams and that excessive overlap should be avoided, the present invention proposes that a large portion (50-80%) of overlap among the individual beams is necessary in order to achieve uniform ablated areas and a smooth profile without ridges. Furthermore, a low-power UV laser, (0.05-1)mJ on the corneal surface, at its bare-beam profile without any beam reshaping is sufficient to achieve a smooth ablation surface based on the method proposed in the present invention, where computer-controlled beam overlap and rotation are employed. In addition to the surface quality problems, it is also impossible for L""Esperance U.S. Pat. No. 4,665,913 to achieve any meaningful clinical results using his proposed techniques if a laser of only (2-4) mJ is available.
Therefore, another object of the present invention is to provide a new method-of beam scanning which combines beam overlap and rotation in a random distribution fashion on the ablated corneal surface such that the individual beam profiles are not critical, where the focused beam has a spot size of 0.1-0.8 mm at a very low energy level 0.05-1 mJ and at its bare-profile is delivered onto the corneal surface in an average fashion. Uniform, near flat-top ablated areas of 1-10 mm in diameter can be performed by the nonuniform starting-beam, but only when a set of specific predetermined overlap and orientation parameters are used. Portions of the theoretical background was published by the inventor, J. T. Lin, in SPIE Pro. vol. 1644, Ophthalmic Technologies II (1991), pp. 266-275.
One of the essential feature of the present invention for the photorefractive surgery processes is to use a scanning device in a laser system which has high repetition rates (50-50,000 Hz), but requires low energy, (0.01-1 mJ per pulse), which is 10 to 100 times less than that of the prior art devices. This new concept enables one to make the refractive lasers at a lower cost, smaller size and with less weight than that of prior art lasers. Furthermore, these compact lasers of the present invention are all solid-state and portable which is particularly suitable for mobile clinic uses. A new concept of UV-laser tissue ablation based on laser peak-power rather than energy is proposed such that lasers at both nanosecond and picosecond pulse duration are suitable for efficient corneal ablation. For lasers with repetition rates lower than 40 Hz, a multi-beam scanning method is proposed in the present invention for efficient ablation.
For ophthalmic applications, it is an aim of the present invention to include but not be limited to photorefractive keratectomy, epikeratoplasty, intrastroma photokeratectomy (IPK), phototherapeutic keratectomy (PTK), and laser in situ keratomileusis (LASIK).
The preferred embodiments of the basic ophthalmic surgery method uses a laser system for the ophthalmic surgery process, including: (1) a diode-pumped solid-state laser of Vanadate crystal (Nd:YVO4) which is frequency-converted by nonlinear crystals of KTP(Potassium titanyl phosphate), LBO (lithium triborate) and BBO(beta barium borate) into the fifth-harmonic at wavelength of 213 nm; (2) a frequency-converted Alexandrite (at high temperature) or Cr:LiSAF with output tunable wavelength of 193-220 nm; (3) a solid-state doubled-YAG pumped, diode-laser injected picosecond Ti:sapphire laser and frequency converted to UV wavelength of 205-215 nm.
According to one aspect of the present invention, the above-described basic lasers with frequency up converted to 193-215 nm focused into a spot size of 0.05-1 mm in diameter, where laser energy per pulse of 0.01-1 mJ is sufficient to achieve the photo-ablation threshold(PAT) energy density of 2-100 mJ/cm2 depending upon the laser parameters (wavelengths and pulse duration). The prior art excimer laser uses large beam spot of 4-6 mm and require much higher laser energy (100-300 mJ) than the low-power lasers presented herein. A software-driven scanning device is used to control the ablation profiles in the present invention, whereas diaphragms or masks are used in the high-power, high-cost excimer laser. In another aspect of the present invention, novel frequency conversion schemes for efficient generation of UV wavelength are proposed including intracavity and optimal-polarization techniques. Another aspect of the invention is temperature and crystal angle-tuning controlled by feedback signals for stable UV output.
A two-dimensional translation device (in X,Y) is integrated into the above laser systems, where the laser heads are compact and light-weight and can be steered to the corneal center by the translation stages rather than moving the patient. The prior art high-powered excimer laser systems are stationary and require a three dimensional adjustable patient""s chair for corneal concentration. Beam steering and scanning is very difficult for these high-power, heavy-weight excimer lasers.
The ophthalmic applications of the laser systems described herein include photorefractive keratectomy(PRK), phototherapeutic keratectomy (PTK), intrastroma photokeratectomy, and laser in situ keratomileusis (LASIK) for myopic, hyperopic, astigmatism and presbyopic corrections.